1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, and more particularly to an evaporative fuel-processing system for internal combustion engines, which is capable of performing abnormality diagnosis of an evaporative emission control system for purging evaporative fuel generated from a fuel tank of the engine into an intake system of same.
2. Prior Art
Conventionally, there has been widely used an evaporative fuel-processing system for internal combustion engines, which comprises a fuel tank, a canister having an air inlet port provided therein, a first control valve arranged across an evaporative fuel-guiding passage extending from the fuel tank to the canister, and a second control valve arranged across a purging passage extending from the canister to the intake system of the engine.
A system of this kind temporarily stores evaporative fuel in the canister, which is then purged into the intake system of the engine.
Whether a system of this kind is normally operating can be checked, for example, by comparing a first value of an air-fuel ratio correction coefficient assumed when purging of evaporative fuel into the intake system is stopped and a second value of the air-fuel ratio correction coefficient assumed when purging of evaporative fuel is effected, after completion of warming-up of the engine. That is, when the evaporative fuel-processing system is normally functioning to purge evaporative fuel into the intake system, an air-fuel mixture supplied to the engine is enriched by the evaporative fuel purged. The enriched air-fuel mixture is detected by an air-fuel ratio sensor, e.g. an O.sub.2 sensor, and hence the air-fuel ratio correction coefficient calculated for feedback control of the air-fuel ratio assumes a smaller value. Therefore, monitoring of the manner of decrease in the air-fuel ratio correction coefficient enables to determine abnormality of the evaporative fuel-processing system. This abnormality diagnosis method is disclosed in U.S. Pat. No. 5,085,194.
However, the above abnormality diagnosis method using the air-fuel ratio correction coefficient suffers from a problem that in the case where a leak of evaporative fuel occurs from defective seals provided at piping connections, valves, the fuel tank, etc. of the system, (e.g. a seal at a filler cap of the fuel tank), it is impossible to detect the leak by the above method, which can result in emission of a large amount of evaporative fuel into the air.